In recent years, as DC consumers such as a DC internet data center (IDC), a DC home, a DC building, and so on have emerged, DC customers have increasingly demanded power supply companies to directly supply DC distribution.
Such DC distribution is a power saving technology. In other words, digitalization of home appliances such as refrigerators, washing machines, and the like as well as existing PCs, TVs, DVDs, and cameras has accelerated because of the development of digital technologies. The power loss in a house arising from power conversion (AC-DC) is 20˜30%. The DC distribution reduces the losses arising from such power conversion, thereby improving energy efficiency and enabling the power supply to be high in quality.
In addition, the DC distribution may be largely classified into medium voltage DC (MVDC) distribution and low voltage DC (LVDC) distribution. In particular, the LVDC distribution has a lower power conversion loss than an AC distribution system in connection to a DC load or a DC power supply, has lower installation and operating costs than the MVDC distribution, and has an advantage that an efficient control of a distribution voltage may be performed by using a power converter.
Meanwhile, power supply companies are studying and developing technologies related to the DC distribution worldwide, and, currently, Korea is also under preparation to construct a demonstration line for the LVDC distribution in an existing network system.
Supply methods of the demonstration line for the LVDC distribution may be composed of a monopolar method and a bipolar method. The bipolar method among both methods may increase a supply capacity and supply reliability. In addition, the bipolar method may stably supply power even when consumer capacities supplied through each pole are different, and even when a pole fails, a sound pole may normally supply DC power.
For a system structure of the bipolar method, an AC/DC converter that provides DC power supply from the DC distribution line plays an important role. Accordingly, research on a structure composed of two converters is underway in order to develop an AC/DC converter capable of providing stable power supply even in case of unbalanced loads wherein the load capacities supplied through each pole are different and in case of an occurrence of a fault at a pole.
However, in order to install a converter at an actual electric line site, the converter should be not only easy to install at the site but also compact and economical. For this connection, a single-type converter composed of one converter is preferable to an AC/DC converter composed of two converters, as the converter of the bipolar method for DC distribution electric lines. That is, in the case of the AC/DC converter composed of two converters, because it is difficult to utilize the existing pole/ground transformer, and thus necessary to reinstall a three-winding transformer, it is not easy to use the AC/DC converter for the current distribution electric line. On the other hand, the single-type converter is economical, is capable of being compact, and has good bidirectional power transmission and output quality.
A method to eliminate the load unbalance situation has been proposed, but a conventional single-type converter has difficulty in providing DC power supply normally to a sound pole when a fault occurs at a pole, whereby a current limit is applied for protection coordination.
Accordingly, in regard to the conventional single-type converter, there is a need for a method capable of providing normal DC distribution when a fault occurs at a pole, whereby a current limit is applied for protection coordination.